Our interest is in the catalytic mechanism of type I DNA topoisomerases, in particular E. coli DNA topoisomerases I and III. We have been studying the interactions of topoisomerase I with DNA. We have collected several data sets from crystals either soaked or co-crystallized in the presence of different oligo- and mono-nucleotides. We have also collected high resolution data on a fragment of topoisomerase I that forms the top part of the molecule. We are interested in the changes that occur in the molecule during different stages of the reaction and this fragment may illustrate the open conformation of the molecule. We also collected some preliminary data on crystals of the RNA component of B. subtilis RNase P. The crystals diffract to 4A resolution at CHESS.